Collisions between large commercial vehicles and much smaller passenger vehicles are often catastrophic. The reconstruction of heavy vehicle accidents requires highly specialized investigative and analytical processes often not needed in the reconstruction of passenger vehicle accidents.
Trucks and buses share three common characteristics that make traffic accident analysis challenging.
Heavy vehicles:
- often vastly out weigh the other vehicle, and therefore conservation of momentum theory is compromised
- are rigid and therefore defy traditional crush damage analysis
- are predisposed to rollover
As an investigator for the National Transportation Safety Board Mr. Painter participated in major accident investigations and studies involving trains, heavy trucks, buses and commercial vehicle operator fatigue.
What was the stopping capability of the vehicle?
Even well maintained heavy vehicles will often require twice the distance as a passenger vehicle in order to brake to a stop. Passenger vehicles will usually skid to a stop from a given speed on a given roadway in a very narrow range of distances, with all four tires locking and skidding approximately the same distance. Establishing deceleration rates and speeds for these hydraulically braked passenger vehicles may be relatively simple, depending upon the accident sequence. However, commercial vehicle air brake systems are much more complicated. Under heavy load, each of the ten brakes on a typical truck tractor semi-trailer combination unit may not be capable of locking down into a full skid. Heavy truck tires may only provide about 75 to 80 % of the stopping force of passenger car tires, even when in full locked skid. Passenger vehicle brakes are self adjusting. However, most air brake systems in operation today require routine maintenance and manual adjustment as the brake linings wear thinner after each application. In such a situation it is preferable to conduct a post accident examination of each of the ten brakes in detail, document the brake adjustment levels, and compute an individual braking force for each brake. Only then can the reconstructionist determine a narrow parameter of likely braking forces and resultant stopping capability of the specific vehicle.
Why did the driver lose control?
Not only do heavy vehicles require a much greater stopping distance than passenger vehicles, they are also much more prone to loss of control. Weight and braking imbalances may create unbalanced forces resulting in jackknifing or trailer swing out into oncoming lanes of traffic. The high center of mass of a loaded heavy truck may result in rollover at speeds safe for a passenger vehicle. Many of these vehicle handling issues can be analyzed with computer simulation techniques as well as hand calculations of rollover threshold and dynamic weight shift. Driver performance issues, to include the influences of adequate training, drugs and fatigue, need also be considered.